1. Field of the Invention
The invention relates to a water heating system, and more particularly to an electric water heating system having a control system for controlling the amount of heat transfer between the electric heat element and the water by modulating the current to the heating element to supply potable water at a substantially constant, controlled temperature.
2. Background of the Related Art
Hot water temperature control devices have conventionally included heat exchangers to accomplish heat transfer between water which rapidly flows within tubes and a heat source, either steam or gas, exposed to the outside of the tubes. Other hot water temperature control devices have used electric heating elements to heat the water. Often a thermostat is employed to establish a temperature set point and to control the heating of the water in such a manner as to maintain the temperature of the water as delivered by the system at this set point.
These systems, generally termed xe2x80x9cinstantaneousxe2x80x9d, do not operate entirely satisfactorily in a water heating system which may have fluctuating flow and input energy. For potable water heaters, the demand fluctuates greatly over a period of time. Demand on the system may remain low for a long period of time, and increase to a higher level at certain times of the day, e.g., in the mornings. As heated (hot) water is withdrawn from the tank, and replaced by unheated (cold) water, the temperature sensor initially detects the desired temperature and suddenly, as the unheated water reaches the sensor, detects a temperature that may be significantly below the set point temperature. The thermostat then calls for more heat to be supplied to the water, but by that time much of the storage tank is filled with unheated water.
This problem is not solved by placing the temperature sensor near the incoming water. Such placement does not permit precise control of the temperature of the water near the outlet of the tank as the water is to be delivered to the appliances or other users of hot water. Moreover, regardless of the location of the sensor in prior art systems of this type, there is an abrupt change in the temperature of the water at the sensor, as the water is withdrawn from the storage tank. Since the system has no way of anticipating this abrupt change, it fails to call for heat as early as it should, and, when a call for heat is made, the system must run at maximum capacity for an extended period in order to catch up with the demand. As a result of such poor temperature control, storage tanks are usually employed for use with an instantaneous system to store heated water at a fixed temperature; in one embodiment water is pumped at a constant rate through the system to keep the temperature constant. Other methods include heating the stored water without pumping means and relying on natural convection to accomplish temperature control.
This problem was previously addressed in commonly-assigned U.S. Pat. No. 4,305,547 (the xe2x80x9c""547 patentxe2x80x9d). While the water heating system disclosed in the ""547 patent was a substantial improvement over the prior art, the present invention seeks to go even further and provides an electric water heating system that provides substantially constant temperature control with reduced complexity and cost to manufacture the system.
An object of the present invention is to provide an electric water heating system which is not subject to significant drops in temperature during periods of increased demand and that provides outlet water at a substantially constant temperature.
Another object of the present invention is to provide an electric water heater which does not require complicated apparatus.
A further object of the present invention is to provide an empirical relationship between the temperature of a mixed flow of inlet water and outlet water and the current applied to the heating element to maintain the setpoint temperature.
These objectives and characteristics are achieved, in accordance with the present invention, by providing a novel combination of several components, including a heat exchanger tank, an inlet flow diverting pipe, an outlet flow diverting pipe, a mixed flow diverting pipe, and a temperature control system.
The heat exchanger tank has an inlet for receiving an inlet flow of liquid into the tank, an outlet for allowing an outlet flow of liquid to leave the tank. The heating element is positioned in the heat exchanger tank and is configured to heat the liquid passing through the water tank from the inlet to the outlet.
The inlet flow diverting pipe carries a portion of the inlet flow diverted from flowing into the tank, and the outlet flow diverting pipe carrying a portion of the outlet flow diverted from the outlet. The mixed flow pipe is configured to carry a flow of liquid partially received from the inlet flow diverting pipe and partially received from the outlet flow diverting pipe. A first tuning valve on the inlet flow diverting pipe is provided to control flow in the inlet diverting pipe. A second tuning valve on the outlet flow diverting pipe is provided to control flow in the outlet diverting pipe.
The temperature control system comprises a temperature sensor configured to determine the temperature of the mixed flow in the mixed flow pipe. The temperature control system modulates the current to the heating element in response to the temperature of the mixed flow based on a predetermined empirical relationship between the current applied to the heating element and the temperature of the mixed flow.
The empirical relationship between the current applied to the heating element and the temperature of the mixed flow has a first data point and a second data point, and may have several intermediate, empirically determined data points. The first data point is defined by the condition wherein the first and second tuning valves are tuned such that the temperature of the mixed flow is a predetermined temperature above the temperature of the inlet flow when the heating element is at substantially maximum current. A second data point is defined by the condition wherein the first and second tuning valves are tuned such that the temperature of the mixed flow is a predetermined temperature below the setpoint temperature when the heating element is at substantially minimum current.
These and other features, aspects, and advantages of the present invention will become better understood with regard to the following detailed description, appended claims, and accompanying drawings.